1. Field of the Invention
This invention relates to avalanche photodiodes and fabrication thereof.
2. Description of the Related Art
In avalanche photodiodes (APDs) photo-carrier density is multiplied by impact ionisation of photogenerated carriers accelerated by a high field. In III-V APD devices for use at wavelengths of 1.3 and 1.5 microns a material such as indium gallium arsenide is used for the region where photogeneration occurs. If the multiplication also takes place in this material, which has a comparatively small band gap, high dark currents resulting from tunnelling occur at the high fields required for multiplication, giving rise to excessive noise. To avoid this problem the holes are photogenerated in indium gallium arsenide and swept into a wider band gap indium phosphide layer containing the pn junction where avalanche multiplication then takes place. This type of device is known as the separate absorption and multiplication (SAM) structure and is well known. The thickness and doping level of the indium phosphide multiplication layer has to be carefully con, rolled in order to achieve the correct field for avalanche multiplication near the pn Junction with a sufficiently low field at the interface with the indium gallium arsenide layer to avoid tunnelling. Both planar and mesa APD structures are known, and in general planar structures have provided superior stability and reliability. In planar devices the pn junction is of necessity curved, and, in order to prevent edge breakdown at the curved edge a p type guard ring is provided around the active part of the device, and it is necessary to carefully control the distribution of the p type dopant in the guard ring.
Various growth methods such as LPE, MOVPE and VPE are available to obtain, in a controlled manner, the layer thicknesses that APD device layers require. Dopants are subsequently diffused or ion implanted into the layers. However, diffusion techniques tend to suffer from lack of reproducibility and flexibility due to the difficulty in achieving exact control over the dopant vapor concentration while implantation techniques require complex and expensive equipment. Also, in the case of sealed ampoule diffusion, for example, the size of the sample that can be processed is limited by the apparatus, such as the maximum practical size of the ampoule.